1. 1 i247: Information Visualization and Presentation Marti Hearst Perceptual Principles

2. 2 Today Visual and Perceptual Principles Which Visual Elements to Use for What?

3. 3 Visual Principles Vision as Knowledge Acquisition Pre-attentive Properties Gestalt Properties Sensory vs. Arbitrary Symbols Relative Expressiveness of Visual Cues

4. 4 Vision as Knowledge Acquisition Perception as a Constructive Act –What you see is not necessarily what you get Adaptation of vision to different lighting situations Image aftereffects Optical illusions Ambiguous figures

5. 5 Vision as Knowledge Acquisition Perception as Modeling the Environment –Evolutionary purpose –When you close your eyes, the world doesn’t disappear! –Examples: Visual completion Object occlusion Impossible objects

6. 6 Vision as Knowledge Acquisition Perception as Apprehension of Meaning –Classification –Attention and consciousness

7. 7 Slide adapted from Stone & Zellweger

8. 8 Physical WorldVisual SystemMental Models Lights, surfaces, objects Eye, optic nerve, visual cortex Red, white, shape Stop sign STOP! StimulusPerceptionCognition External World

9. 9 Slide adapted from Stone & Zellweger Visual System Light path –Cornea, pupil, lens, retina –Optic nerve, brain Retinal cells –Rods and cones –Unevenly distributed Cones –Three “color receptors” –Concentrated in fovea Rods –Low-light receptor –Peripheral vision From Gray’s Anatomy

10. 10 Slide adapted from Stone & Zellweger Cone Response Encode spectra as three values –Long, medium and short (LMS) –Trichromacy: only LMS is “seen” –Different spectra can “look the same” Sort of like a digital camera* From A Field Guide to Digital Color, © A.K. Peters, 2003

11. 11 Slide adapted from Stone & Zellweger Eyes vs. Cameras Cameras –Good optics –Single focus, white balance, exposure –“Full image capture” Eyes –Relatively poor optics –Constantly scanning (saccades) –Constantly adjusting focus –Constantly adapting (white balance, exposure) –Mental reconstruction of image (sort of) http://www.usd.edu/psyc301/ChangeBlindness.htm

12. 12 Slide adapted from Stone & Zellweger

13. 13 Slide adapted from Stone & Zellweger

14. 14 Slide adapted from Stone & Zellweger Color is relative

15. 15

16. 16 Interference RED GREEN BLUE PURPLE ORANGE Call out the color of the letters

17. 17 Interference PURPLE ORANGE GREEN BLUE RED Call out the color of the letters

18. Preattentive Processing A limited set of visual properties are processed preattentively –(without need for focusing attention). This is important for design of visualizations –What can be perceived immediately? –Which properties are good discriminators? –What can mislead viewers?

19. Example: Color Selection Viewer can rapidly and accurately determine whether the target (red circle) is present or absent. Difference detected in color. From Healey 97 http://www.csc.ncsu.edu/faculty/healey/PP/index.html

20. Example: Shape Selection Viewer can rapidly and accurately determine whether the target (red circle) is present or absent. Difference detected in form (curvature) From Healey 97 http://www.csc.ncsu.edu/faculty/healey/PP/index.html

21. Pre-attentive Processing < 200 - 250ms qualifies as pre-attentive –eye movements take at least 200ms –yet certain processing can be done very quickly, implying low-level processing in parallel If a decision takes a fixed amount of time regardless of the number of distractors, it is considered to be preattentive.

22. 22 Demonstration 13579345978274055 24937916478254137 23876597277103866 19874367259047362 95637283649105676 32543787954836754 56840378465485690 Time proportional to the number of digits 13579345978274055 24937916478254137 23876597277103866 19874367259047362 95637283649105676 32543787954836754 56840378465785690 Time proportional to the number of 7’s 13579345978274055 24937916478254137 23876597277103866 19874367259047362 95637283649105676 32543787954836754 56840378465785690 Both 3’s and 7’s seen preattentively Count the 7’s

23. 23 Contrast Creates Pop-out Hue and lightnessLightness only

24. 24 Pop-out vs. Distinguishable Pop-out –Typically, 5-6 distinct values simultaneously –Up to 9 under controlled conditions Distinguishable –20 easily for reasonable sized stimuli –More if in a controlled context –Usually need a legend

25. Example: Conjunction of Features Viewer cannot rapidly and accurately determine whether the target (red circle) is present or absent when target has two or more features, each of which are present in the distractors. Viewer must search sequentially. From Healey 97 http://www.csc.ncsu.edu/faculty/healey/PP/index.html

26. 26 Let’s try it! http://www.csc.ncsu.edu/faculty/healey/PP/index.html

27. Example: Emergent Features Target has a unique feature with respect to distractors (open sides) and so the group can be detected preattentively.

28. Example: Emergent Features Target does not have a unique feature with respect to distractors and so the group cannot be detected preattentively.

29. Asymmetric and Graded Preattentive Properties Some properties are asymmetric –a sloped line among vertical lines is preattentive –a vertical line among sloped ones is not Some properties have a gradation – some more easily discriminated among than others

30. Use Grouping of Well-Chosen Shapes for Displaying Multivariate Data

31. SUBJECT PUNCHED QUICKLY OXIDIZED TCEJBUS DEHCNUP YLKCIUQ DEZIDIXO CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC GOVERNS PRECISE EXAMPLE MERCURY SNREVOG ESICERP ELPMAXE YRUCREM CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM GOVERNS PRECISE EXAMPLE MERCURY SNREVOG ESICERP ELPMAXE YRUCREM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC SUBJECT PUNCHED QUICKLY OXIDIZED TCEJBUS DEHCNUP YLKCIUQ DEZIDIXO CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC

32. SUBJECT PUNCHED QUICKLY OXIDIZED TCEJBUS DEHCNUP YLKCIUQ DEZIDIXO CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC GOVERNS PRECISE EXAMPLE MERCURY SNREVOG ESICERP ELPMAXE YRUCREM CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM GOVERNS PRECISE EXAMPLE MERCURY SNREVOG ESICERP ELPMAXE YRUCREM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC SUBJECT PUNCHED QUICKLY OXIDIZED TCEJBUS DEHCNUP YLKCIUQ DEZIDIXO CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC Text NOT Preattentive

33. Preattentive Visual Properties (Healey 97) length Triesman & Gormican [1988] width Julesz [1985] size Triesman & Gelade [1980] curvature Triesman & Gormican [1988] number Julesz [1985]; Trick & Pylyshyn [1994] terminators Julesz & Bergen [1983] intersection Julesz & Bergen [1983] closure Enns [1986]; Triesman & Souther [1985] colour (hue) Nagy & Sanchez [1990, 1992]; D'Zmura [1991] Kawai et al. [1995]; Bauer et al. [1996] intensity Beck et al. [1983]; Triesman & Gormican [1988] flicker Julesz [1971] direction of motion Nakayama & Silverman [1986]; Driver & McLeod [1992] binocular lustre Wolfe & Franzel [1988] stereoscopic depth Nakayama & Silverman [1986] 3-D depth cues Enns [1990] lighting direction Enns [1990]

34. 34 Slide adapted from Tamara Munzner Gestalt Principles Idea: forms or patterns transcend the stimuli used to create them. –Why do patterns emerge? –Under what circumstances? Principles of Pattern Recognition –“gestalt” German for “pattern” or “form, configuration” –Original proposed mechanisms turned out to be wrong –Rules themselves are still useful

35. Gestalt Properties Proximity Why perceive pairs vs. triplets?

36. Gestalt Properties Similarity Slide adapted from Tamara Munzner

37. Gestalt Properties Continuity Slide adapted from Tamara Munzner

38. Gestalt Properties Connectedness Slide adapted from Tamara Munzner

39. Gestalt Properties Closure Slide adapted from Tamara Munzner

40. Gestalt Properties Symmetry Slide adapted from Tamara Munzner

41. Gestalt Laws of Perceptual Organization (Kaufman 74) Figure and Ground –Escher illustrations are good examples –Vase/Face contrast Subjective Contour

42. 42 Unexpected Effects

43. 43 Emergence Holistic perception of image Slide adapted from Robert Kosara

44. More Gestalt Laws Law of Common Fate –like preattentive motion property move a subset of objects among similar ones and they will be perceived as a group

45. 45 Influence on Visualization Why we care –Exploit strengths, avoid weaknesses –Optimize, not interfere Design criteria –Effectiveness –Expressiveness –No false messages

46. 46 Design criteria: Effectiveness Faster to interpret More distinctions Fewer errors 01234567 This? Or this?

47. 47 Sensory vs. Arbitrary Symbols Sensory: –Understanding without training –Resistance to instructional bias –Sensory immediacy Hard-wired and fast –Cross-cultural Validity Arbitrary –Hard to learn –Easy to forget –Embedded in culture and applications

48. 48 American Sign Language Primarily arbitrary, but partly representational Signs sometimes based partly on similarity –But you couldn’t guess most of them –They differ radically across languages Sublanguages in ASL are more representative –Diectic terms –Describing the layout of a room, there is a way to indicate by pointing on a plane where different items sit.

49. Which Properties are Appropriate for Which Information Types?

50. Interpretations of Visual Properties Some properties can be discriminated more accurately but don’t have intrinsic meaning (Senay & Ingatious 97, Kosslyn, others) –Density (Greyscale) Darker -> More –Size / Length / Area Larger -> More –Position Leftmost -> first, Topmost -> first –Hue ??? no intrinsic meaning –Slope ??? no intrinsic meaning

51. 51 Rankings: Encoding quantitative data Cleveland & McGill 1984, adapted from Spence 2006

52. 52 Which properties used for what? Stephen Few’s Table: AttributeQuantitativeQualitative Line length 2-D position Orientation Line width Size Shape Curvature Added marks Enclosure Hue Intensity

53. 53 Next Week Your homework assignment Tufte readings How to Critique